Digian cuts path for portable audio players without analog circuitry

YOKOHAMA CITY, Japan ( ChipWire/EET) -- A Japanese design house here said it has developed a digital 1-bit audio amplifier that can be used to design portable audio players without the need for a D/A converter or analog amplifier.

Based on a pulse-width modulation signal conversion scheme that's used for high-end audio formats in Japan, Digian Technology said its technology will allow customers to significantly reduce the silicon area required in portable systems and can easily be ported to a 3.3-volt process technology.

The announcement comes on the heels of Sharp Corp.'s announcement that it has developed its own 1-bit digital audio amplifier based on the same PWM scheme used in the Super Audio CD (SACD) format pioneered by Sony and Philips (see Sept. 3 story). The Sharp device is set to hit the U.S. market next month.

"Sharp is introducing it's amplifier for the high-end, but we're targeting low-end devices," said Yoshiaki Shinohara, a former mixed-signal circuit designer with Burr-Brown Japan who founded Digian in January 1998.

Though true, Sharp expects its product to eventually trickle down into portable music players, car stereos, PCs and TVs. Both companies said they realize what full digital audio processing can offer in terms of high integration, scalability and relative ease in porting to different process technologies -- attributes that are still elusive with mixed-signal design.

In Japan, where commuter trains are crammed with people listening to MiniDisk players tucked in their pockets, the potential market is already a reality. And with major Japanese consumer electronics companies getting ready to debut their first MP3 players this year, that will only increase the market potential, Shinohara said.

Digian's solution is based on a mish-mash of digital circuits, from a front-end receiver link to an output buffer. In a prototype system, a 16-bit optical digital signal from a CD player was fed to a toss-link interface and routed through an interface receiver. In alternative schemes for audio sources such as MP3 and MiniDisk, the compressed digital audio signals are decoded to 16-bit signals.

These signals are fed to a digital filter and then, at the most crucial point in its processing, the signal is converted to a low-bit digital stream by a delta-sigma modulator. While still in digital form, the signal then passes through a pulse-width modulator (PWM), which then converts it into a one-bit signal.

Next, the signal passes through a standard 74 series buffer, which drives the headphone speakers as low as 20 milliwatts. The higher-frequency noise is filtered out using low-pass filters between the buffer and speakers, each with a coil and capacitor, before outputting to a pair of 8-ohm headphone speakers that produce stereo sound. The low-pass filters have the equivalent functionality of a D/A converter, and the acoustic energy depends on the duty of the pulse.

As a result of this scheme, the audio system has a signal-to-noise rating of 90 dB, a total harmonic distortion (plus noise) of 0.03 percent and an 80 percent efficiency rating, according to Digian.

Because of its low driveability, the digital-only audio amplifer is limited to headphone speakers. But it is these portable audio systems that can most take advantage of the on-chip integration offered by digital CMOS. Analog circuitry, by contrast, is less amenable to integration with digital logic since its performance depends on process technology. In addition, analog circuitry is not as a scalable as digital designs, and is subject to signal degradation caused by digital noise coming from metal lines and the circuit substrate.

Shinohara said Digian uses techniques that are well known in the esoteric world of analog circuit design, but nevertheless are not easy to implement. While the application is considered low-end, Digian is using a sixth order delta-sigma conversion scheme that is "technically difficult but necessary to get higher performance," he said. Delta-sigma conversion is the core technology of D/A converters widely used in digital audio products; most use third to fifth order delta-sigma conversions.

Delta-sigma conversion seeks to suppress audible noise in the 0- to 20-kHz audio range, which is the generally accepted range perceivable by humans. The order of delta-sigma modulation is equivalent to the number of integrators in the circuit. When the number of integrators is small, the noise level is higher in the audible range and increases gradually. When the order is higher, the noise level is kept at a minimum in the audible range, and then increases rapidly beyond 20 kHz, Shinohara explained. Thus at the sixth order, which represents the top-end of most delta-sigma converters, most of the noise can be pushed out into the inaudible frequency range.

Sixth-order delta-sigma has been achieved by a handful of other companies specializing in mixed-signal audio devices, such as Cirrus Logic Corp.'s Crystal Semiconductor subsidiary and by Sharp, Shinohara said. Sharp has reached a new watermark with its latest multichip 1-bit digital amplifier, in which Sharp uses a seventh-order delta-sigma conversion device to realize a S/N ratio of almost 120 dB and a frequency response of 0-100 kHz.

When designing such high-order delta-sigma converters, the most common place to get tripped up is in the feedback loops, which are needed to improve the in-band noise floor, according to Digian.

"Delta-sigma conversion is not easily understood by the average engineer," said Shinohara. "And in Japan, there are only about three design houses that have good audio expertise."

Digian finished developed its digital audio amplifer in July, and has a demonstration board based on a Xilinx Inc. field-programmable gate array (FPGA). The company is now getting ready to offer it to Japanese customers in RTL format, which should make it easy to perform logic synthesis. The delta-sigma converter and PWM and other circuitry take up about 6,000 gates of logic, which places a single-chip solution within the reach of today's process technology, Shinohara said.